A work machine such as a hydraulic excavator can exhibit excellent combined operability by connecting a boom directional control valve and a swing directional control valve to the same pump line in parallel and driving a swing motor and a boom cylinder by a common pump. The “excellent combined operability” means a characteristic that, in a case of, for example, a so-called swing boom raising operation for simultaneously implementing swing and boom raising, a swing acceleration becomes lower as a boom raising operation amount is larger, compared with a case of a sole swing operation. This characteristic results from the higher inertia of a swing structure than the inertia of a boom, and the characteristic is obtained since a swing load pressure is higher than a boom load pressure at initial swing to allow a more hydraulic fluid to flow into the boom cylinder. Under this characteristic, if a swing distance is short relative to a height of a soil discharge position, a boom raising speed increases but a swing increasing rate decreases as the boom raising operation amount is larger, so that the soil discharge position is advantageously easy to adjust during gravel loading work or the like. On the other hand, if the boom load pressure differs from the swing load pressure, a divergence loss can be generated in response to a difference in the load pressure.
Meanwhile, there is known a work machine configured such that a swing motor and a boom cylinder are driven by different pumps and that a swing speed decreases during a swing boom raising operation (refer to Patent Document 1 and the like). Specifically, the work machine is configured such that a delivery rate control valve for controlling a delivery rate of each pump is used to cause a delivery pressure of the pump corresponding to the boom cylinder to act on the delivery rate control valve of the pump corresponding to the swing motor when a boom raising operation is detected, and to decrease a supply flow rate for the swing motor at a time of a swing boom raising operation.